There are two types of widely used fluxing technology in the electronics industry: liquid spray and liquid foaming. For both techniques, to achieve a uniform deposition, the solvent in the flux must wet the electronic board to be soldered to form a continuous film. Conventionally, the volume of the solvent needed to wet a board is relatively large. But for a soldering operation having no post-soldering cleaning steps, the flux residue after the soldering must be benign and minimum in quantity. Accordingly, most no-clean fluxes usually have very high solvent content (95% to 99%) and low solid content (typically 1% to 5%).
There are basically two types of solvents in use today: volatile organic compounds (VOC) which evaporate easily during the soldering process, and water. Low solid content fluxes using alcohol or other organic compounds as solvents wet the board easily. However, they emit large amounts of VOC during the soldering process and thus create environmental problems. Low solid content water-based fluxes, on the other hand, need a surfactant to assist wetting, since water has a very high surface tension when deployed upon the electronic board and metals to be joined. The surfactant leaves a hygroscopic residue after the soldering operation and thus has to be cleaned off or the final product has to be protected with a conformal coating or an encapsulant.
The quest for better ways to clean precision electronic components without ozone-depleting solvents had led to the development of cleaning processes that reduce the need for solvents. Techniques have now emerged for using supercritical carbon dioxide instead of environmentally harmful CFC-based solvents to remove particles and organic contaminants introduced during the manufacturing of circuit boards. However, the use of a supercritical carbide dioxide may tend to adversely attack the board itself or a plastic housing which may accommodate the board. Accordingly, for these among other reasons, supercritical carbon dioxide may be a sub-optical approach to cleaning electronic circuit boards. Carbon dioxide becomes supercritical--that is, it remains as a gas but has the properties of liquid organic solvents--at relatively low pressures and temperatures. When heated to 31.degree. C. and 73 bars (1050 psi), carbon dioxide is in a supercritical state and possesses the properties of a liquid solvent. In a supercritical state, it is highly diffusive and its low surface tension allows it to penetrate into small spaces to dissolve residues completely from the complex surfaces of manufactured parts. Supercritical carbon dioxide is nontoxic and nonflammable.
U.S. Pat. No. 5,013,366 discloses supercritical CO.sub.2 as a solvent to clean organic contaminants, including soldering flux residue. U.S. Pat. No. 5,288,332 discloses CO.sub.2 dissolved in water to remove metal contamination. U.S. Pat. No. 4,566,916 discloses CO.sub.2 produced through decomposition of CaCO.sub.3 during the welding operation to dilute H.sub.2, N.sub.2 and O.sub.2 partial pressure during welding. U.S. Pat. No. 3,275,201 discloses CO.sub.2 being used as a propellant for a pressurized flux package. However, that disclosure teaches operation at low pressures (15 to 40 psi), which is much lower than a supercritical pressure (1050 psi). At this pressure range, CO.sub.2 cannot be used as a solvent for a flux.